The present invention relates to optical fibers and their alignment, and more particularly, to a method and apparatus for actively aligning optical fibers to optical devices.
In practical fiber optic systems, transmission of light between optical fibers and any optical device requires careful alignment and tight tolerances. A mismatched junction at the ends of two connected optical fibers may cause loss of transmitting light. When some light is lost, such loss causes attenuation of the signal to be transferred.
Referring to FIG. 1, an exploded view is provided for illustrating a typical multi-fiber connector. A multi-fiber connector 100 has an upper plate 101 and a lower plate 103 each of which has V-grooves 104, 105 on its surface. To connect the first set of optical fibers 107 with the second set of optical fibers 109, the optical fibers 107, 109 are slipped into the respective V-grooves 105 on the lower plate 103 from the opposite directions. Then, the upper plate 101 with the V-grooves 104 each matching with the respective V-grooves 105 of the lower plate 103 is applied on the top of the optical fibers 107, 109. As a result, the ends of two optical fibers are mated with one another in corresponding V-grooves 104, 105.
A limitation in conventional multi-fiber connectors employing V-grooves 104, 105 is the need for tight tolerances in the V-grooves 104, 105. This limitation will be described in detail with reference to FIG. 2A.
FIG. 2A is a cross-sectional view of the multi-fiber connector 100 in FIG. 1. Each of the optical fibers 201 is placed in a space formed by the corresponding V-grooves 104, 105 of the upper and lower plates 101, 103. The conventional multi-fiber connector 100 also employs adhesive material 115 to fix the optical fibers 201 between the V-grooves 104, 105 and to attach the upper and lower plates 101, 103 to each other. Each of the optical fibers 201 maintains physical contact with the corresponding V-grooves 104, 105. Thus, the shape and dimension of the matching V-grooves 104, 105 determine the position of an optical fiber held between the matching V-grooves 104, 105.
FIG. 2B illustrates the connecting section of a typical planar waveguide device 220 comprising a planar substrate 210 and a plurality of optical waveguides 211 disposed thereon. This connecting section can be mated with a multi-fiber connector to couple light from each of the waveguides 211 into each corresponding optical fiber 201. The mating of the connecting section and the multi-fiber connector is typically used to introduce light into the waveguides 211, or to retrieve light therefrom. In a typical use, the waveguides 211 will transmit the light to, or return the light from, an optical device (not shown) that has been formed on the substrate 210.
As previously noted, any deviation in the alignment of optical fibers affects light transmission of the optical fibers. Such deviation or inaccuracy of alignment and positions of the optical fiber cores is mainly caused by factors as follows:
First, the optical fiber cores may be misaligned due to unevenly applied pressure on the upper substrate. When the upper and lower substrates are bonded to each other by pressing the upper substrate toward the lower substrate, pressure applied on the upper substrate should be maintained to have the same force over the entire area. Since positions of the optical fiber cores are determined after bonding the upper and lower substrates to each other, unevenly or incompletely applied pressure on the upper substrate may cause misalignment of the optical fiber cores.
Second, misalignment of the optical fiber cores may also be caused by an error in forming the V-grooves on the substrates. The V-grooves should be spaced relative to each other and should have a predetermined distance (or height) from the bottom of the lower plate so that the optical fibers arranged on the V-grooves may be fixed at their target positions. If there is any error in the space between the V-grooves and/or the distance from the bottom of the lower plate, the optical fibers arranged on the V-grooves may deviate from the target positions so as to cause misalignment of the optical fiber cores.
Finally, the optical fiber cores may be misaligned due to an error in their concentricity. It is assumed in fabrication of the optical fiber arrays that an optical fiber core is centered on the corresponding optical fiber. However, in fabrication of optical fibers, concentricity of the optical fiber cores may be failed. In this case, the optical fiber cores may be misaligned even though pressure on the upper substrate is evenly applied and the V-grooves are properly spaced each other and have a predetermined depth.
Further, in the configuration illustrated in FIG. 2B, there may be some error in the position of each of the waveguides on the planar waveguide device. When the multi-fiber connector is mated with the planar waveguide device the error in the waveguide position will create an error in the coupling of the waveguide to the corresponding optical fiber, and create a loss of optical signal.
Therefore, there remains a need for a method of aligning multiple optical fibers with high accuracy so that the optical fiber cores are precisely aligned with optical devices or waveguides disposed in connection with the optical fibers.
In a first aspect of the present invention a method for actively aligning optical fibers to optical waveguides. The method includes providing a base substrate on which the optical devices are arranged; forming on the base substrate thermal pads each of which is disposed in alignment with corresponding one of the optical devices; depositing a bonding agent on the respective thermal pads; selectively activating the thermal pads so that selected thermal pads generate heat to melt the bonding agent thereon; placing the optical fibers on the respective thermal pads, optical fibers on the selected thermal pads are surrounded by the melted bonding agent; adjusting the optical fibers on the selected thermal pads to be aligned to the respective optical devices; and solidifying the bonding agent on the selected thermal pads by inactivating a heat source directed to the selected thermal pads.
In one embodiment, the selectively activating step includes providing electrical connections between a current source and the respective thermal pads; and controlling connection and disconnection of the electrical connections based on alignment status between each optical fiber and a corresponding optical waveguide. In another embodiment, the step of forming the thermal pads includes forming heating pads on the base substrate; providing a first electrical connection between a current source and the respective heating pads, wherein the heating pads are commonly connected to the first electrical connection; and providing a second electrical connection between the current source and the respective heating pads, each of the heating pads being connected to the second electrical connection through a switching device.
In a second aspect of the present invention, an apparatus for aligning a plurality of optical fibers to a plurality of optical waveguides comprises a base substrate, thermal pads formed on the top surface of the base substrate and connected to a current source, and solder deposited on the respective thermal pads, wherein each of the optical fibers aligned to a corresponding optical waveguide is surrounded by the solder on corresponding one of the thermal pads, wherein the thermal pads melt the solder thereon when current is provided to the thermal pads from the current source.
In one embodiment, each of the thermal pads is preferably connected to the current source through a switching device for controlling supply of current to a corresponding thermal pad, wherein the switching device may be turned on when a corresponding optical fiber is adjusted to be aligned to a corresponding optical waveguide, and turned off when the corresponding optical fiber is aligned to the corresponding optical waveguide.
In an alternative embodiment, the apparatus includes positioners for adjusting positions of the respective optical fibers in response to a first feedback signal; switches for controlling supply of current to the respective thermal pads in response to a second feedback signal, light detectors associated with the respective optical waveguides, each of which detects light transmitted through corresponding one of the optical waveguides, power meters each for measuring the amount of light detected by corresponding one of the light detectors; and a computer for receiving outputs from the respective power meters and providing the first feedback signal to the respective positioners and the second feedback signal to the respective switches.